The present invention relates to an exposure control device and an electronic camera on which the exposure control device is mounted.
In general, in order to obtain proper exposure for shooting with an electronic camera, EV (exposure value) is obtained by the following APEX (Additive System of Photographic Exposure) operation.EV=TV+AV=BV+SV  (1)
In the expression (1), TV denotes a shutter speed defined in an APEX unit, and is 0 when the actual shutter speed is 1 second, and increases in increments of 1 each time the shutter speed increases by one step. AV denotes an aperture defined in the APEX unit, and is 0 when the actual aperture is f/1.0, and increases in increments of 1 each time the aperture is reduced by one step. BV denotes a brightness of an object defined in the APEX unit. SV denotes the exposure sensitivity defined in the APEX unit.
The words “proper exposure” means that the aperture and the shutter speed are adjusted such that the exposure at the brightest point of an object image on an imaging surface of an image pickup device and the exposure at the darkest point of the object image on the imaging surface of the image pickup device fall within an exposure range of the image pickup device. Therefore, for the same ISO sensitivity, it is necessary to make an adjustment so that the distribution of the exposure on the imaging surface of the image pickup device constantly falls within the same exposure range regardless of the brightness of the object image. When the ISO sensitivity is determined, a proper exposure range is determined. Then, the shutter speed is adjusted so that the brightness distribution of the object image matches the proper exposure range.
In the electronic camera, if the exposure sensitivity SV can be changed, the aperture AV, the shutter speed TV and the exposure sensitivity SV are calculated in accordance with the object brightness BV. Japanese Patent Provisional Publication No. 2003-189175A discloses an exposure control device configured to change the exposure sensitivity to obtain the proper exposure. In the exposure control device disclosed in the publication, the exposure EV is calculated from the object brightness BV and the set exposure sensitivity SVs. Then, a control aperture AVc and a control shutter speed TVc are determined based on the calculated exposure EV. Furthermore, an exposure deviation ΔEV is determined from the above described exposure EV, the aperture AVc and the shutter speed TVc, and then it is judged whether the exposure is proper based on the value of ΔEV. When the exposure is not proper, a control exposure sensitivity SVc is obtained using the set exposure sensitivity SVs and the value of ΔEV. By using the obtained control exposure sensitivity SVc in place of the exposure sensitivity SVs, it becomes possible to perform photo shooting while setting the proper exposure.